What I want to do

I want to realize this on Drupal9.

• Finally,I want to create the custom field on a webform.
• With this custom field,Submitter who use the webform can add a new taxonomy term to a vocabulary (This vacabulary is predetermined in module) on this webform.
• This field is text field.
• If the new term already exits in the vocaburaly,it isn’t added to the vocabulary.
• If it is possible,the submitter can also choose a term from the terms which is already registered.

Problem

I am trying custom field which can register a term in a content type. (I can’t add custom field to webform.So,I tried it first.) I already created it tentatively.However,if I write something to this field,a warning is output.

Warning: Array to string conversion in Drupalmymodule2PluginFieldFieldTypeMyModule2Field->isEmpty() (line 62 of modules/custom/mymodule2/src/Plugin/Field/FieldType/MyModule2Field.php).

MyModule2 is the custom module which I created.

So I want to know how to create a custom field which can register a term properly.

Code and Folder Structure

mymodule2(folder) -mymodule2.info.yml

-mymodule2.module

-src(folder)/Plugin(folder)/Field(folder)

Field(folder)-FieldType(folder)-MyModule2Field.php

Field(folder)-FieldWidget(folder)-MyModule2Widget.php

Field(folder)-FieldFormatter(folder)-MyModule2Formatter.php

Codes

mymodule2.info.yml

name: mymodule2 type: module description: hogehoge core_version_requirement: ^8.8 || ^9 package: Example 

mymodule2.module

<?php ?> 

MyModule2Field.php

    <?php  namespace Drupalmymodule2PluginFieldFieldType;  use DrupalCoreFieldFieldItemBase; use DrupalCoreFieldFieldStorageDefinitionInterface; use DrupalCoreTypedDataDataDefinition; use DrupalCoreStringTranslationTranslatableMarkup; use DrupaltaxonomyEntityTerm;       use DrupalCoreLink; use DrupalCoreUrl; /**  * Plugin implementation of the 'MyModule2' field type.  *  * @FieldType(  *   id = "mymodule2",  *   label = @Translation("MyModule"),  *   description = @Translation("This field is used to store alpha-numeric values."),  *   default_widget = "MyModule2Widget",  *   default_formatter = "MyModule2Formatter"  * )  */ class MyModule2Field extends FieldItemBase {    /**   * {@inheritdoc}   */      public static function propertyDefinitions(FieldStorageDefinitionInterface $definition) {     // Prevent early t() calls by using the TranslatableMarkup.     $properties['mymodule2'] = DataDefinition::create('string')     ->setLabel(new TranslatableMarkup('Text value'));       return $properties;   }     /**    * {@inheritdoc}    */   public static function schema(FieldStorageDefinitionInterface $definition) {     $schema = [         'columns' => [           'mymodule2' => [             'type' => 'varchar',             'length' => 255,           ],         ],     ];                    return $schema;   }     /**    * {@inheritdoc}    */   public function isEmpty() {     $value = $this->getValue();     if (isset($value['mymodule2']) && $value['mymodule2'] != '') {              /*this is problem code*/       $val=implode("",$value);       $arr = explode(',',$val);       $categories_vocabulary='pixelarttag';       foreach($arr as $a){         $term = term::create(array(         'parent' => array(),         'name' => $a,         'vid' => $categories_vocabulary,))->save();       }       return FALSE;     }     return TRUE;   }   } ?> 

MyModule2Widget.php

<?php  namespace Drupalmymodule2PluginFieldFieldWidget;   use DrupalCoreFieldFieldItemListInterface; use DrupalCoreFieldWidgetBase; use DrupalCoreFormFormStateInterface;   /**  * Plugin implementation of the 'MyModule2Widget' widget.  *  * @FieldWidget(  *   id = "MyModule2Widget",  *   label = @Translation("My Field widget"),  *   field_types = {  *     "mymodule2"  *   }  * )  */ class MyModule2Widget extends WidgetBase {    /**    * {@inheritdoc}    */   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {         $element['mymodule2'] =  [       '#type' => 'textfield',       '#title' => 'AddictionTaxonomy',       '#description' => 'Custom field to be used for alpha-numeric values',       '#default_value' => isset($items[$delta]->title) ? $items[$delta]->title : NULL,       '#weight' => 0,     ];       return $element;   }   } ?> 

MyModule2Formatter.php

<?php namespace Drupalmymodule2PluginFieldFieldFormatter;   use DrupalCoreFieldFormatterBase; use DrupalCoreFieldFieldItemListInterface;   /**  * Plugin implementation of the 'MyModule2Formatter' formatter.  *  * @FieldFormatter(  *   id = "MyModule2Formatter",  *   label = @Translation("My Field Formatter"),  *   field_types = {  *     "mymodule2"  *   }  * )  */ class MyModule2Formatter extends FormatterBase {    /**    * {@inheritdoc}    */   public static function defaultSettings() {     return [       // Implement default settings.     ] + parent::defaultSettings();   }     /**    * {@inheritdoc}    */   public function settingsSummary() {     $summary = [];     $summary[] = $this->t('Displays the random string.');     return $summary;   }     /**    * {@inheritdoc}    */   public function viewElements(FieldItemListInterface $items, $langcode) {       $elements = [];     foreach ($items as $delta => $item) {       $elements[$delta] = [         '#type' => 'markup',         '#markup' => $item->mymodule2       ];     }       return $elements;   } } ?> 

P.S.

I am not good at English, and I also do not know much about Drupal.I have studied Drupal since 1 month ago.So,I might ask something stupid.Please forgive me

Sincerely